"She was so calm about it," Cromwell says. "I honestly just became sick to my stomach and at first I didn't know if she'd ever be on our team again."

Montgomery was determined to keep running.

"She said, 'I want to run. I want to run fast. And I don't want you to hold back,' " Cromwell says. "This was more than just running fast, this was a journey in trying to keep Kayla one step ahead of MS."

Montgomery has become one of the country's fastest young distance runners. She's ranked 21st in the nation after winning the North Carolina state title in the 3,200 meters in February.

"When the race first starts, I feel everything. I can feel my legs moving and I can feel the start of pain and after reaching the first mile marker, I've started to lose most feeling in my legs," Montgomery says. "The momentum is kind of what keeps my legs moving and once I stop, they just kind of fall off from underneath me."

Coach Cromwell waits at the finish line at every race to catch Kayla.

"With one lap to go, I run across the track and just get ready to catch her," Cromwell says. "We catch her just to protect her. We don't want her to brace for a fall and break an arm or collarbone or something."

After her diagnosis, Montgomery feared the day she would no longer be able to run.

"For a few years, I was terrified that I might not be able to run tomorrow or the next day," Montgomery says. "I kind of decided that that wasn't really helping me and I wasn't happy living like that. So I stopped focusing on the what-ifs, and [started] focusing on what I'm able to do now. Making sure that I make the most of that and take the gift of mobility and use it to the greatest advantage I can."